Conventional helicopter designs have been dominated by those which connect the engine to a single main rotor assembly by a transmission. The transmission permits the engine(s) (whether piston or gas turbine) to run at its optimal speed while reducing the rotational speed of the rotor to ensure optimal performance of the main rotor.
The twisting of the shaft that drives the main rotor creates an equal and opposite reaction that twists the fuselage or body of the helicopter in the opposite direction thereby requiring the use of a tail rotor, fenestron or system utilizing the Coanda effect in order to maintain directional stability.
In the case of a tail rotor and a fenestron, when the pilot pushes on one of the rudder pedals, the blade angle of the tail rotor changes producing more or less thrust and induces aircraft yaw. In the case of a system utilizing the Coanda effect, a rudder pedal input also changes the pitch of the internal fan blades increasing or decreasing the air flow through the duct which is then ejected through slots on the side of the tail boom inducing yaw.
In helicopters that do not require a tail rotor, other solutions for maintaining directional control must be devised.